Subscriber-side telephone terminal equipment (TE) can be connected to a telephone network at a so-called subscriber ("S") interface. Over the "S" interface, time-division multiplexed voice and data can be serially communicated. The subscriber's TE is synchronized to the data received at the "S" interface to provide frame alignment and to provide synchronization of each bit position within a channel's time-slot. Such synchronization must occur rapidly and should minimize discontinuities in the generation of a synchronization timing (clock) signal, "jitter", during start-up to avoid information loss and error generation.
It is desirable to afford a free-running clock signal even when there is no traffic over the "S" interface, such as when the line is "dead", when sychronization with the received data has been lost, or for purely local communications and yet automatically enter into synchronization with any traffic at the "S" interface.
Known in the prior art are complex synchronization schemes employing phase-locked-loop circuitry which can achieve rapid synchronization with data at the "S" interface in a relatively "coarse" fashion; multiple-bit adjustments in the phase of the generated clock signal occur during clock recovery. Such discontinuities can produce erratic behavior in the circuits receiving the generated clock signals and may require lenghty waiting periods for the clock to stabilize during which no useful function is performed by these circuits.
Accordingly there is a need for a serial port synchronizer which refines the clock signal recovered by the coarser methods so as to operate in a free-running mode during periods of no activity or clock-acquisition at the serial port and performs a smooth, relatively jitter-free, synchronization with the acquired clock once it has been locked by the coarser method.